In Relation to Semiconductor Microlithography
Ongoing efforts to produce computers of higher speed at lower cost has led to the search for more efficient methods of fabricating high-resolution, high-density integrated circuitry on semiconducting substrates such as doped silicon and gallium arsenide. One aspect of that search is the investigation of methods for producing patterns of high resolution, i.e., patterns having line widths of less that one micron (a micron is one millionth of a meter), an area of research known as microcircuit lithography. For a detailed description of this subject, see the book entitled "Introduction to Microlithography", L. F. Thompson, C. G. Willson, and J. J. Bowden, editors, ACS Press, N.Y. (1983). At the present rate of miniaturization of integrated circuitry, it is anticipated that a resolution of approximately one quarter of a micron (i.e. 0.25 u) will be required within the next decade. For a discussion of the current state of the art in microlithography and an assessment of future requirements, see the monograph entitled, "The Submicron Lithography Labyrinth", A. N. Broers, Solid State Technology, June 1985, pp. 119 to 126.
In conventional fabrication of integrated circuits, patterning of the semiconductor surface is accomplished using the following general procedure. A radiation-sensitive organic coating (a "resist") is applied to the wafer surface. Prior treatment of the wafer with an adhesion-promoter such as hexamethyldisilazane (HMDS) is often employed. The coated surface is exposed to patterned radiation such as light, electron beams, gamma rays or X-rays. Exposure is made either by the "flood" or by the "scanning beam" technique. In flood irradiation, all the regions to be irradiated are exposed simultaneously. Patterned radiation is achieved either by projecting an image onto the substrate or by interposing a mask between the light source and the substrate. In the beam technique, the work is broken into small regions, or "pixels" that are exposed sequentially, generally by causing the beam to trace out the desired pattern. A "positive" resist material is one in which the irradiated region becomes more soluble, for example, by photo-induced bond scission. A "negative" resist material is one in which the irradiated region becomes less soluble, generally due to a free-radical polymerization reaction. Chemical development (e.g., exposure to concentrated sodium hydroxide or chlorinated hydrocarbon solvent) leaves behind a pattern of insoluble organic material. Exposure to an ion plasma or etchant solution removes substrate material in the uncovered areas. Residual organic material is chemically stripped, revealing the etched "troughs" and the unetched "plateau" regions that were protected by the resist.
Some of the prime considerations in the commercial production of integrated microelectronic circuitry are: resolution of the features in the semiconductor substrate; throughput; uniformity and reproducibility; and capital equipment and materials cost. Beam lithographic techniques that involve irradiation with electrons, ions, X-rays, gamma rays, or ultraviolet (UV) are known to offer resolution of features in the organic resist layer of well below one micron. For example, 0.4 micron features have been produced in poly(methylmethacrylate) (PMMA) using sub-200 nm (a nanometer is one thousandth of a micron or one billionth of a meter) UV irradiation from an excimer laser (D. Ehrich, et. al., J. Vac. Sci., 1985). Electron beam irradiation of multilayer films of vinyl stearate and w-tricosenoic acid, deposited using the Langmuir-Blodgett technique, has produced 60 nm wide lines and spaces (see: A. Barraud, et. Al., Thin Solid Films, 68, 1980, pp.91-100; also, A. Broers and M. Pomerantz, Thin Solid Films, 99, 1983, pp. 323-329).
A number of drawbacks exist with beam lithographic techniques. First, computer-controlled beam systems require considerable capital expenditure and are expensive to maintain. Second, the sequential irradiation of individual pixels is far more time-consuming than flood irradiation techniques. Throughput considerations (i.e., the time required to produce the item) take on greater significance as feature density and wafer size both continue to increase. Third, there is a tradeoff between resolution in the resist and etch resistance. It is known that the energy lost by electrons beamed into a solid is scattered in an oblong pear-shaped volume of diameter roughly equal to the electron penetration depth. The penetration depth increases with the energy of the incident electrons. Consequently, the diameter of the exposed area is at a minimum when the penetration depth is equal to the film thickness (positing the requirement that the entire thickness of the film be irradiated). Therefore, a means of obtaining improved resolution is by use of a thinner resist film, such as spin-cast organic polymer films or the Langmuir-Blodgett films described previously. However, ultra-thin organic film resists suffer from a number of problems that include film inhomogeneity (particularly pinholes) and the inability to withstand the vigorous plasma etching processes used to transfer the features of the resist to the underlying substrate.
Optical flood lithographic processes are the most widely employed because they offer the best combination of resolution and throughput. At the present time, the limit of resolution of microcircuitry features that can be produced on a scale practical for commercial production is on the order of one micron. Optical lithography generally involves patterned UV (sub-400 nm) irradiation of semiconductor substrates coated with a spin-cast organic resist film that is usually 300 nm to one micron thick. Principal limitations to attainment of higher resolution are due to a combination of the wavelength of light employed, the film composition, and thickness.
In optical lithography, it is known that resolution varies inversely with the wavelength of the irradiation. Therefore, high resolution is achieved by using radiation of the shortest possible wavelength to which the resist is sensitive. A number of light sources suitable for UV irradiation are available, including mercury lamps, xenon lamps, Nd-YAG lasers, excimer lasers, and fourth harmonic generation. Most of the high resolution photopolymeric resists that are now available are sensitive to near-UV (i.e., 320 to 400 nm) light. Few, if any photoresists are sensitive in the deep-UV (200 to 320 nm) or the vacuum-UV (below 200 nm) regions.
The wave length of ultraviolet radiation is in the 4 to 400 nm range. That range is loosely divided into near-UV (400 to 300 nm), far-UV (300 to 200 nm), and extreme-UV (below 200 nm). Extreme-UV radiation is strongly absorbed by air and therefore extreme-UV is usually used in evacuated apparatus. For that reason, extreme-UV is often refered to as "vacuum-UV".
As discussed above for beam techniques, the spin-coated resist films used in optical lithography must be at least several tenths of a micron thick to avoid pinholes and provide adequate resistance to plasma etching. Other limitations to resolution arise with the use of thick films due to defocussing of the image in the film, the occurrence of standing waves in the film, Rayleigh scattering from film inhomogeneities and from reduced control of the spatial extent of photoreactions. Spin coating tends to produce films that are thicker at the edges than in the center. Variations in the thickness of the film causes loss of resolution during contact mask exposure (i.e., where a patterned mask is in direct contact with the resist-coated substrate). Additionally, spin-coating machines are expensive and the substrates must be coated serially (i.e., one after the other).
Once patterned conventional optical resists generally require chemical development of the image (i.e., removal of the soluble resist material). Solvents employed in development, especially chlorinated hydrocarbons, are known to be environmentally hazardous. Resolution degradation is also induced during development by imperfect dissolution of the resist
Other difficulties encountered with known resist films include imperfect or weak adhesion to the substrate, which can render the piece of work useless if needed resist regions come loose from the substrate. Resist materials often require special care in handling due to their sensitivity to ambient light, moisture and temperature.
Fabrication of conductive paths on a semiconductor substrate can be accomplished in a number of ways Generally, a thin metal coating is applied by vapor or sputter deposition over the entire area of the substrate. Most of the metal is removed in a later step following patterning and development steps. No flood optical lithographic process currently exists whereby high resolution metal patterns can be selectively deposited.